Vertical-cavity surface-emitting laser layout for high bandwidth output

ABSTRACT

A layout for a vertical-cavity surface-emitting laser (VCSEL) is provided. In an example embodiment, the layout comprises a VCSEL, an etched shape around a mesa of the VCSEL, a signal contact layer deposited on section of the mesa, and a ground contact layer. The ground contact layer comprises three parts and is positioned around a first section of the etched shape. The first part of the ground contact layer is deposited on a second section of the etched shape. The second and third parts of the ground contact layer comprise two legs off of the first part. The two legs are symmetrically positioned about two sides of the signal contact layer to form a ground-signal-ground configuration.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.62/524,854, filed Jun. 26, 2017, the content of which is incorporatedherein in its entirety by reference.

BACKGROUND

Embodiments of the present invention relate generally to reducingparasitics in vertical-cavity surface-emitting lasers (VSCELs) toincrease the efficiency and bandwidth frequency of the output laser.VSCELs are often used to convert electrical signals to optical signalsfor use in fiber optic data and analog transmission through fiber opticcable systems. Systems and methods are described herein according toembodiments of the present invention that allow for increasedtransmission speeds through the fiber optic cable systems by utilizing anew type of VCSEL layout.

BRIEF SUMMARY

The use of VCSELs for transmission of optical signals in fiber opticsystems has provided several advantages over edge-emitting lasers. Forexample, VCSELs generally require less power consumption and can bemanufactured more efficiently than edge-emitting lasers, especially whenon-chip testing capability is provided, which can result in aconsiderable cost advantage as compared to edge-emitting lasers.Furthermore, VCSELs typically provide reliable operation over time,which can be very important for applications in fiber optic systems.

To meet the continuously growing demands for increased bandwidth intelecommunication networks caused by growing data traffic in big datacenters as well as in local and access networks, the inventors haverecognized a need for a layout of VCSELs on an optical chip whichreduces parasitics (e.g., unwanted capacitance or resistance) of theVCSEL. The reduction of the parasitics thus increases the VCSEL outputfrequency.

Embodiments of the present invention utilize a ground-signal-ground(GSG) layout with an etched shape around the VCSEL mesa, such that thedata transmission capabilities of a VCSEL are not limited by theparasitic capacitance of traditional VCSEL layout designs.

According to a first aspect of the present invention, a layout for aVCSEL is provided. In an example embodiment, the layout for the VCSELcomprises a VCSEL; an etched shape around a mesa of the VCSEL; a signalcontact layer deposited on a section of the mesa of the VCSEL; and aground contact layer. The ground contact layer comprises three partsthat are positioned around a first section of the etched shape. A firstpart of the ground contact layer is deposited on a second section of theetched shape. Second and third parts of the ground contact layercomprise two legs off the first part. The two legs are symmetricallypositioned about two sides of the signal contact layer to form aground-signal-ground configuration.

In an example embodiment, the signal contact layer comprises a probenotch, wherein the probe notch is configured to guide a testing probe.

In an example embodiment, the etched shape is a portion of an annulusabout the mesa of the VCSEL. In an example embodiment, an interior edgeof the annulus has a mesa diameter defined by the mesa and an outer edgeof the annulus has a second diameter defined by the first part of theground contact layer.

In an example embodiment, a first gap defining a first width separatesthe second part of the ground contact layer and the signal contact layerand a second gap defining a second width separates the third part of theground contact layer, the first width being approximately the same asthe second width. In an example embodiment, the first width and thesecond width are between 19 and 20 micrometers.

According to another aspect of the present invention, a layout for aVCSEL is provided. In an example embodiment, the layout for the VCSELcomprises a VCSEL; a signal contact layer deposited on a section of amesa of the VCSEL; and a ground contact layer. The ground contact layerscomprises a first leg portion, a second leg portion, and an arc portion.The arc portion partially encircles the mesa. The first leg portion andthe second leg portion extend symmetrically from opposite ends of thearc portion. The signal contact layer extends between the first legportion and the second leg portion to form a ground-signal-groundconfiguration. The layout for the VCSEL further comprises an etchedshape positioned between the arc portion of the ground contact layer anda corresponding portion of the signal contact layer.

In an example embodiment, the signal contact layer comprises a probenotch, wherein the probe notch is configured to guide a testing probe.

In an example embodiment, the etched shape is a portion of an annulusabout the mesa of the VCSEL. In an example embodiment, an interior edgeof the annulus has a mesa diameter defined by the mesa and an outer edgeof the annulus has a second diameter defined by the arc portion of theof the ground contact layer.

In an example embodiment, a first gap defining a first width separatesthe first leg portion of the ground contact layer and the signal contactlayer and a second gap defining a second width separates the second legportion of the ground contact layer, the first width being approximatelythe same as the second width. In an example embodiment, the first widthand the second width are between 19 and 20 micrometers.

According to still another aspect of the present invention, a layout fora VCSEL is provided. In an example embodiment, the layout for the VCSELcomprises a VCSEL; a signal contact layer deposited on a section of amesa of the VCSEL; and a ground contact layer comprising a first legportion, a second leg portion, and an arc portion. The arc portionpartially encircles the mesa and the first leg portion and the secondleg portion extend symmetrically from opposite ends of the arc portion.The signal contact layer extends between the first leg portion and thesecond leg portion to form a ground-signal-ground configuration. Thesignal contact layer comprises a probe notch, wherein the probe notch isconfigured to guide a testing probe.

In an example embodiment, the probe notch is a V-shaped notch. In anexample embodiment, the notch comprises a notch opening that is 20 to 25micrometers wide. In an example embodiment, the notch extends 10 to 25micrometers into the signal contact layer. In an example embodiment, theVCSEL layout further comprises an etched shape positioned between thearc portion of the ground contact layer and a corresponding portion ofthe signal contact layer.

BRIEF DESCRIPTION OF THE DRAWING(S)

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 illustrates a schematic top view of a typicalground-signal-ground vertical-cavity surface-emitting laser (VCSEL)layout;

FIG. 2 illustrates a schematic side view of a VCSEL according to anexample embodiment;

FIG. 3 illustrates a perspective top view of a ground-signal-groundVCSEL layout according to an example embodiment;

FIG. 4 illustrates a partial cross-section view of aground-signal-ground VCSEL layout according to an example embodiment;

FIG. 5 illustrates a schematic top view of a ground-signal-ground VCSELlayout according to an example embodiment; and

FIG. 6 illustrates a schematic top view of a ground-signal-ground VCSELlayout array according to an example embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which some, but not allembodiments of the inventions are shown. Indeed, these inventions may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout. As usedherein, terms such as “top,” “about,” “around,” etc. are used forexplanatory purposes in the examples provided below to describe therelative position of certain components or portions of components. Asused herein, the term “approximately” refers to tolerances withinmanufacturing and/or engineering standards.

FIG. 1 illustrates a typical ground-signal-ground vertical-cavitysurface-emitting laser (VCSEL) layout 100 on an optical chip. Asdescribed herein, a VCSEL layout comprises the position of a VCSEL andthe position of contact layers connected to a VCSEL on an optical chipor wafer. The optical chip may comprise one or more VCSELs 102 in theVCSEL layout 100 or an array of VCSELs 102 arranged over a plurality ofVCSEL layouts 100. The VCSEL 102 may be configured to output light foruse in a variety of applications, such as fiber optic data transmissionin high-speed fiber optic communication systems.

As shown in FIG. 2 the structure of the VCSEL 102 may include an activeregion 202 disposed between two reflector stacks 204 and 206. The VCSEL102 may also include a mesa structure 208. Contact layers 108 and 106may be provided on either side of the VCSEL 102 and may be configured toconduct electricity through the VCSEL 102, such that light can begenerated in the active region 202 and the reflector stacks 204 and 206and output through the top of the VCSEL 102, as shown in FIGS. 1 and 2.In some examples, the entire structure comprises metallic material toallow for future wire bonding.

Referring back to FIG. 1, the contact layer 106 may serve as anelectrical ground, while the contact layer 108 may serve as theelectrical signal layer that is configured to provide current to theVCSEL 102. For example, providing an electrical current to the contactlayer 108 may energize the VCSEL 102 such that light (e.g., an opticalsignal) is output from the VCSEL 102 while the electrical circuit iscompleted by the contact layer 106.

In the manufacturing process, a photolithography process may be used todefine bond pads including a gap 110 between the contact layer 108 andthe contact layer 106. The process may then include depositing thecontact pads on the bond pads and utilizing a lift off phase to removeexcess conducting material.

Through hard work and applied ingenuity, the inventors have discovered anew layout design of a VCSEL in which the parasitics of the VCSEL layoutdescribed herein are reduced as compared with existing VCSEL designs. Insome examples, the VCSEL layout 400 shown in FIG. 3 and described belowreduces the parasitic properties of the VCSEL layout, such that afrequency of the optical signal output from the VCSEL can increase from20 gigahertz (GHz) to 30 GHz. In some examples, with furtherimprovements the frequency of the optical signal output may approach alimit of 70 GHz.

FIGS. 3-6 illustrate a ground-signal-ground VCSEL layout according toexample embodiments. Specifically, FIG. 3 illustrates a top perspectiveview of a VCSEL layout 400 and FIG. 4 provides a cross-section of anexample embodiment of the VCSEL layout 400 illustrated in FIG. 3. TheVCSEL layout 400 includes a VCSEL 402, a first contact layer 408, asecond contact layer 406, and a gap 410 between the first and secondcontact layers 406 and 408. In an example embodiment, the first contactlayer 408 is a signal contact layer and the second contact layer 406 isa ground contact layer. In some examples, the material making up thecontact layers may comprise gold. In some examples, the VCSEL 402 issimilar to the VCSEL 102 described in FIG. 2. For example, an exampleembodiment of the VCSEL 402 comprises may include an active region 452disposed between two reflector stacks 454 and 456. The active region 452and the reflector stacks 454 and 456 may define the mesa structure 414.For example, the mesa structure 414 may comprise the active region 452sandwiched between the reflector stacks 454 and 456. The first contactlayer 408 may encircle the VCSEL opening 460 such that the first contactlayer 408 defines an aperture of the VCSEL 402 having a first diameterd1 and through which the light 1 is emitted from the VCSEL. For example,the first contact layer 408 may at least partially extend around the topof the mesa structure 414. In an example embodiment, the first diameterd1 is approximately 22.398 micrometers (μm). For example, the firstdiameter d1 may be 21-24 μm.

The second contact layer 406 may extend in an arc around the VCSEL 402at a second diameter d2 that is greater than the second diameter d1. Inan example embodiment, the second diameter d2 is approximately two tofour times greater than the first diameter d1. In the illustratedembodiment of FIG. 3, the first contact layer 408 serves as theelectrical signal layer that is configured to provide current to theVCSEL 402, and the second contact layer 406 serves as the electricalground to complete the electrical circuit. The layout 400 may alsoinclude an etched section including an etched shape 412 about a mesa 414of the VCSEL 402. In an example embodiment, the etched shape 412 may bea portion about the mesa 414 that has been etched down to a dielectricand/or insulating material 458. In an example embodiment, the insulatingmaterial 458 is Benzocyclobutene (BCB). In an example embodiment, theVCSEL 402 is formed on a substrate 460. For example, an arc-shapedtrench may be etched about the mesa structure 414 between the firstcontact layer 408 and the second contact layer 406. In some examples,this process includes standard metallization in the P-N contact layer.For example, Ti/Pt/Au may be used for the P contact layer andGe/Ni/Pt/Ti/Au may be used for the N contact layer.

In some examples, the layout for the VCSEL 400 comprises the VCSEL 402and the etched shape 412 around the mesa 414 of the VCSEL 402. Thelayout 400 may also include the first contact layer 408 deposited on asection of the mesa 414 of the VCSEL 402. The layout may also includethe second contact layer 406. In various embodiments, the second contactlayer 406 comprises three parts or portions. A first part 406 c of thesecond contact layer 406 may be deposited on a section of the etchedshape, and a second part 406 a and a third part 406 b of the secondcontact layer 406 may each comprise a leg extending from the first part.For example, the second contact layer 406 may comprise an arc portion,such as the first part 406 c, and leg or extended portions, such as thesecond and third parts 406 a and 406 b. In an example embodiment, thefirst part 406 c is an arc portion defined by the second diameter d2. Inan example embodiment, the legs (second and third parts 406 a and 406 b)extend from opposite ends of the arc portion (the first part 406 c) awayfrom the arc portion such that a first end of each leg is at leastpartially in contact (e.g., in electrical contact) with the arc portion(the first part 406 c) and a second end of each leg is disposed at adistal position with respect to the arc portion. The two legs orextended portions (the second and third parts 406 a and 406 b) may besymmetrically positioned about two sides of the first contact layer 408to form a ground-signal-ground configuration. In an example embodiment,the first and second contact layers 408, 406 may be generally M-shaped.For example, the second and third parts 406 a and 406 b of the secondcontact layer 406 may provide the outer legs of the M shape, the firstcontact layer 408 may provide the middle leg of the M shape, and thefirst part 406 c of the second contact layer 406 may provide anintermediate arched or bent portion connecting the two outer legs of theM shape.

In particular, the second part 406 a and the third part 406 b of thesecond contact layer 406 may be configured such that a distal end 420 ofeach (e.g., an end that is distal for the first part 406 c) is widerthan portions of the second and third parts 406 a, 406 b that are moreproximal to the first part 406 a. In this way, distal ends 411 of thegap 410 extend towards each other at the distal end 420 of the secondand third parts 406 a, 406 b of the second contact layer 406. Forexample, the distal end 413 of the first contact layer 408 is the end ofthe first contact layer 408 that is distal to the mesa 414. The distalend 413 of the first contact layer 408 may be tapered. For example, thesecond and third parts 406 a and 406 b (e.g., the legs and/or extendedportions of the second contact layer 406) may widen starting at aninflection point 560 and continue to widen toward the distal ends 420thereof and the first contact layer 408 may narrow or taper starting atthe inflection point 560 and continue to narrow toward the distal end413 of the first contact layer. In an example embodiment, the second andthird parts 406 a and 406 b (e.g., the legs and/or extended portions ofthe second contact layer 406) the taper or narrow between the first part406 c and the inflection point. For example, the width of the second andthird parts 406 a and 406 b (e.g., the legs and/or extended portions ofthe second contact layer 406) may be smaller at the inflection point 560compared to where the second and third parts 406 a and 406 b extend fromand/or contact the first part 406 c of the second contact layer 406. Inan example embodiment, the width of the second and third part 406 a and406 b may be approximately constant between the first part 406 c and theinflection point 560. In an example embodiment, the inflection point 560is located closer to the mesa 414 of the VCSEL layout 402 than to thedistal ends 420, 413 of the first and second contact layers 408, 406. Inan example embodiment, the first part 406 c may comprise an arc portionand the second and third parts 406 a, 406 b, may comprise a first legportion and a second leg portion that extend symmetrically from oppositeends of the arc portion.

The inventors have determined that the layout shown in FIG. 3 providesmore current to the VCSEL 402 than standard layouts and reduces theamount of current that would be parasitically drained through othermaterials in the VCSEL layout 400. In particular, the “M”-like shapeprovided by the second contact layer 406 and the first contact layer 408extending therebetween (e.g., between the a second part 406 a and athird part 406 b) reduces the parasitics of the VCSEL 402. The increasedcurrent sent directly to the VCSEL 402 provides the increased frequencyof the optical signal output from the VCSEL 402. In some examples, theVCSEL 402 also provides low cross talk between neighboring VCSELs suchas shown in the VCSEL array 600 depicted in FIG. 6. The VCSEL 402 mayalso provide low noise levels and facilitate testing through the use ofa notch 502 through pattern recognition for standard testing of theVCSEL.

As shown in FIGS. 3-6, the VCSEL layout 400 includes the etched shape412. As described above, the etched shape 412 further decreases theparasitic materials around the mesa of VCSEL 402, to lower the parasiticcapacitance of the VCSEL layout 400. In some examples, the etched shape412 comprises an arc and/or a half or semi-circle shape, such as a “C”shape around the mesa of the VCSEL 402. For example, the etched shape412 may be a portion of an annulus defined by the first diameter d1 orby the mesa diameter d3 at the interior edge of the annulus and thesecond diameter d2 at the outer edge of the annulus.

As shown in FIG. 4, the first part 406 c of the second contact layer 406and the etched shape 412 may comprise a stepped feature such that thethickness of the VCSEL is smaller proximate the mesa 414 than at otherpositions of the VCSEL layout 402. For example, the distance between thesubstrate 460 and the exterior surface of the VCSEL may be smaller atthe etched shape 412 than at other portions of the VCSEL (e.g., in thegap 410, the first part 406 c of the second contact layer 406, thesecond or third parts 406 a, 406 b of the second contact layer 406, thefirst contact layer 408). In an example embodiment, the etched shape 412extends around at least half of the perimeter of the mesa 414 (e.g., atleast half way around the circumference defined by d3). In an exampleembodiment, the etched shape 412 extend around greater than halfperimeter of the mesa 414 (e.g., approximately 60% of the way around66.67% of the way around, 75% of the way around, and/or the like).

Furthermore, in some examples, the VCSEL layout 400 is symmetrical suchthat the gap 410 on both sides of the first contact layer 408 andbetween the first contact layer 408 and the second contact layer 406 arethe same shape and distance on both sides of an axis 550. For example,the width of the gap 410 between the first contact layer 408 and thelegs of the second contact layer 406 may be the same on both sides ofthe first contact layer 408. The gap 410 may, for example, have a width510. In some examples, the width 510 is approximately 19.23213 μm. Inother examples, the width 510 may be between 19 μm and 20 μm. In any ofthe above examples, the gap 410 may be the same on both sides of thefirst contact layer 408. For example, a gap 410 may be disposed betweenthe second part 406 a (e.g., a first leg) of the second contact layer406 and the first contact layer 408 such that the gap 410 separates thesecond part 406 a (e.g., a first leg) of the second contact layer 406from the first contact layer 408 by a first width. Similarly, a gap 410may be disposed between the third part 406 b (e.g., a second leg) of thesecond contact layer 406 and the first contact layer 408 such that thegap 410 separates the third part 406 b (e.g., a second leg) of thesecond contact layer 406 from the first contact layer 408 by a secondwidth. The first and second width may be approximately equal.

In another example embodiment, the VCSEL layout 400 includes a notch 502in the first contact layer 408. The notch 502 may be a probe notchconfigured to provide a guide for a testing probe in the testing stageof VCSEL manufacturing. The notch 502 may thus allow the probe to moreeasily make contact with the first contact layer 408, such that theprobe can provide a testing electrical current to the VCSEL 402. In anexample embodiment, the notch 502 is a V-shaped notch; however, variousother notch shapes are considered. In an example embodiment, the notch502 has an opening that is 20 to 25 μm wide and that is disposed on anedge or surface of the first contact layer 408. In an exampleembodiment, the notch extends 10 to 25 μm into the first contact layer408.

FIG. 6 illustrates a VCSEL array 600 on an optical chip, including oneor more of the VCSEL layouts 400 for a plurality of VCSELs.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the inventions are not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

That which is claimed:
 1. A contact structure for a vertical-cavitysurface-emitting laser (VCSEL) comprising: a VCSEL; an etched trencharound a mesa of the VCSEL, the etched trench having an interior edgelocated proximate the VCSEL, a distal edge spaced from the VCSEL, and atrench bottom extending between the interior edge and the distal edge; asignal contact layer deposited on a section of the mesa of the VCSEL andat least partially encircling the VCSEL via a signal contact layer arcand extending a length away from the mesa to a distal end, wherein awidth of the signal contact layer decreases from an inflection pointline to the distal end of the signal contact layer, the inflection pointline located along the length of the signal contact layer and beingtransverse to a direction defined by the length of the signal contactlayer; and a ground contact layer comprising three parts: a first partof the ground contact layer deposited on a second arc positioned arounda first arc, wherein (a) the first arc comprises the interior edge and afirst portion of the trench bottom, the first portion of the trenchbottom being adjacent the interior edge, and (b) the second arc (i) isconcentric with the first arc and (ii) comprises a second portion of thetrench bottom and the distal edge, the second portion of the trenchbottom being adjacent the distal edge, and a second part of the groundcontact layer, and a third part of the ground contact layer, wherein:each of the second and third parts of the ground contact layer comprisesa leg extending from a respective end of the second arc and inelectrical communication with the first part to a respective distal end,a width of each of the second and third parts of the ground contactlayer increases from the inflection point line to the respective distalend of each of the second and third parts, and the legs aresymmetrically positioned about two sides of the signal contact layer toform a ground-signal-ground configuration; wherein the first arcseparates the signal contact layer from the first part of the groundcontact layer.
 2. The contact structure of claim 1, wherein the signalcontact layer comprises a probe notch, wherein the probe notch isconfigured to guide a testing probe.
 3. The contact structure of claim1, wherein the etched trench is a portion of an annulus about the mesaof the VCSEL.
 4. The contact structure of claim 3, wherein an interioredge of the annulus has a mesa diameter defined by the mesa and an outeredge of the annulus has a second diameter defined by the first part ofthe of the ground contact layer.
 5. The contact structure of claim 1,wherein a first gap defining a first width is disposed between thesecond part of the ground contact layer and the signal contact layer anda second gap defining a second width is disposed between the third partof the ground contact layer and the signal contact layer, the firstwidth being approximately the same as the second width.
 6. The contactstructure of claim 1, wherein the signal contact layer arc is defined bya first diameter, an interior edge of the first arc is defined by asecond diameter; and the second diameter is at least approximately twotimes the first diameter.
 7. The contact structure of claim 1, whereinthe first part of the ground contact layer comprises a step, the step isdisposed on the second portion of the trench bottom and adjacent thedistal edge of the etched trench, and the first part of the groundcontact layer of the step is thicker than a remainder of the first partof the ground contact layer disposed on the second portion of the trenchbottom.
 8. The contact structure of claim 1, wherein the first arc andthe second arc do not overlap.
 9. The contact structure of claim 1,wherein the first arc is devoid of any part of the ground contact layeror the signal contact layer.
 10. A contact structure for avertical-cavity surface-emitting laser (VCSEL) comprising: a VCSEL; asignal contact layer deposited on a section of a mesa of the VCSEL andat least partially encircling the VCSEL via a signal contact layer arc;and a ground contact layer comprising a first leg portion, a second legportion, and an arc portion, the arc portion partially encircling themesa, and the first leg portion and the second leg portion extendingsymmetrically from opposite ends of the arc portion, wherein the signalcontact layer extends between the first leg portion and the second legportion to form a ground-signal-ground configuration; and an etchedtrench positioned between the arc portion of the ground contact layerand a corresponding portion of the signal contact layer, the etchedtrench having an interior edge located proximate the VCSEL, a distaledge spaced from the VCSEL, and a trench bottom extending between theinterior edge and the distal edge, wherein: a first arc comprises theinterior edge and a first portion of the trench bottom, the firstportion of the trench bottom being adjacent the interior edge, a secondarc is concentric with the first arc and comprises a second portion ofthe trench bottom and the distal edge, the second portion of the trenchbottom being adjacent the distal edge, the arc portion of the groundcontact layer is deposited on the second arc, the first arc is devoid ofany part of the ground contact layer or the signal contact layer, andthe first arc separates the signal contact layer from the arc portion ofthe ground contact layer, the signal contact layer extends a length awayfrom the mesa to a distal end, a width of the signal contact layerdecreases from an inflection point line to the distal end of the signalcontact layer, the inflection point line is located along the length ofthe signal contact layer and is transverse to a direction defined by thelength of the signal contact layer, and each of a width of the first legportion and a width of the second leg portion increases from theinflection point line to a respective distal end of each of the firstleg portion and the second leg portion.
 11. The contact structure ofclaim 10, wherein the signal contact layer comprises a probe notch,wherein the probe notch is configured to guide a testing probe.
 12. Thecontact structure of claim 10, wherein the etched trench is a portion ofan annulus about the mesa of the VCSEL.
 13. The contact structure ofclaim 12, wherein an interior edge of the annulus has a mesa diameterdefined by the mesa and an outer edge of the annulus has a seconddiameter defined by the arc portion of the ground contact layer.
 14. Thecontact structure of claim 10, wherein a first gap defining a firstwidth is disposed between the first leg portion of the ground contactlayer and the signal contact layer and a second gap defining a secondwidth is disposed between the second leg portion of the ground contactlayer and the signal contact layer, the first width being approximatelythe same as the second width.
 15. A contact structure for avertical-cavity surface-emitting laser (VCSEL) comprising: a VCSEL; asignal contact layer deposited on a section of a mesa of the VCSEL, thesignal contact layer comprising a proximate portion that encircles, atleast in part, a VCSEL opening of the VCSEL, and a distal end, thedistal end being spaced apart from the VCSEL by an intermediate portionof the signal contact layer extending between the proximate portion andthe distal end; a ground contact layer comprising a first leg portion, asecond leg portion, and an arc portion, the arc portion partiallyencircling the mesa, and the first leg portion and the second legportion extending symmetrically from opposite ends of the arc portion,wherein the intermediate portion of the signal contact layer extendsbetween the first leg portion and the second leg portion from theproximate portion to the distal end to form a ground-signal-groundconfiguration, and a probe notch formed completely through the distalend of the signal contact layer in a direction substantially parallel toan emission direction of the VCSEL, wherein the probe notch isconfigured to guide a testing probe.
 16. The contact structure of claim15, wherein the probe notch is a V-shaped notch.
 17. The contactstructure of claim 15, wherein the probe notch comprises a notch openingthat is 20 to 25 micrometers wide.
 18. The contact structure of claim15, wherein the probe notch extends 10 to 25 micrometers into the signalcontact layer.
 19. The contact structure of claim 15, further comprisingan etched trench positioned between the arc portion of the groundcontact layer and a corresponding portion of the signal contact layer.